Lactate production has been measured in isolated segments of rat nephron. Production rates were determined under aerobic and anaerobic conditions. Rates obtained when oxidative metabolism was inhibited by antimycin A were used to evaluate the glycolytic capacity of each segment during anoxia. In the next series of experiments ammonia production will be measured in individual nephron segments of potassium depleted and potassium loaded rats. The aim is to determine in which sites of the nephron ammoniagenesis is altered in response to in vivo changes of potassium balance.